Many kinds of transducers have been conceived to convert a variety of physical effects into a corresponding movement of a transducer element with respect to one or more other fixed elements. The relative position of the movable element is a measure of the physical effect. For example, pressure transducers are responsive to an input fluid pressure to deflect the movable element.
While it would be highly desirable to sense the position of the movable element directly, small changes in miniature transducers are often at the atomic level and thus not measurable with practical equipment. A common method of determining the position of the movable element is to measure the electrical capacitance between it and the fixed elements. Sophisticated circuits are utilized to very accurately transform the capacitance into corresponding electrical signals which are representative of the pressure, acceleration, etc., to which the transducer is responsive.
Typical capacitive displacement type transducers include two capacitors, one (C1) existing between a first plate and the movable element, and the second capacitor (C2) existing between a second plate and the movable element. As the movable element moves toward one plate or the other in response to input changes, the capacitances between the movable element and the two plates change accordingly. Therefore, by measuring the capacitances between the movable element and the plates, the amount of deflection can be measured. This nominally balanced structure provides a natural zero (equal capacitances with no input) and a structure which responds to inputs of either polarity.
The trend in transducer manufacture, at least with the capacitive displacement type transducer, is to reduce the size of the device. This is due primarily to the fabrication of silicon structures according to current silicon processing techniques. Hence, by reducing the size of the structure, many more devices can be fabricated from a silicon wafer, thereby decreasing the cost. However, smaller structures result in smaller transducer capacitances and smaller changes of capacitance in response to input changes, the result of which must be accurately sensed by associated electrical circuits.
The measurement of the transducer capacitances must be made independently of the unavoidable extraneous stray capacitances between the transducer elements and the environment, which do not respond to the input. As transducer devices become miniaturized, the stray capacitance becomes very significant, often being of the same magnitude as the capacitance of the transducer capacitors C1 and C2 being measured. This can be appreciated in view that with miniature transducers, the total capacitance may be in the neighborhood of several picofarads, with significant changes in the capacitance due to input changes being a matter of femtofarads.
Further, traditional transducer circuits introduce additional parasitic parameters, such as input capacitance, which can have an increasing effect on accuracy as the capacitance of the transducer is reduced.
Further still, traditional transducer circuits often produce an output which is proportional to the difference of the transducer capacitances, (C1-C2). This function produces an output which is very non-linear for significant deflections of the movable element, since the deflection is approximately proportional to the reciprocal of capacitance. This leads to complex linearization corrections or the restriction of the deflection of the movable element, with a resulting reduction of sensitivity.
Further still, traditional transducer systems are subject to many sources of scaling errors, wherein the sensitivity is affected by various parameter variations. For example, the transducer capacitances are directly proportional to the dielectric constant of the fluid between the capacitor elements. A change of the dielectric constant, caused by a change of fluid type, humidity, temperature or other variables, will change the sensitivity of the transducer. Also, the sensitivity is affected by changes of the dimensions of the transducer, such as caused by thermal expansion or contraction, since the capacitances are dependent on the area of the capacitance elements and spacings between them. In addition, the electrical output resulting from the transducer capacitances is often directly related to the values of resistors, capacitors and other components in the electronics which exhibit instabilities and temperature sensitivities. This leads to the use of precision and costly components.
A method for eliminating the effects of stray capacitance from the transducer elements to the environment and parasitic circuit capacitance is disclosed in U.S. Pat. No. 4,584,885. However, the output of the described circuits is proportional to (C1-C2) and is thus non-linear for significant displacements. Since the described application involves a force-feedback system, in which the movable element experiences no displacement, non-linearity from this source is not an issue. However, in any transducer wherein the output is a function of the displacement, this source of non-linearity is critical. Further, the method described in the noted patent does not address the effects of variation of scaling factors in the transducer or the circuit on the system.
It can be seen that a need exists for a transducer system in which a miniature transducer and associated circuit provides enhanced linearity and is not sensitive to stray capacitance from the transducer elements to the environment, dielectric constant variations, scaling of the transducer dimensions, parasitic circuit parameters and circuit component value variations. Another need exists for a transducer circuit which is cost effective and easily manufactured.